1. Field of the Invention:
This invention relates to high voltage semiconductor switches and method of making the same.
2. Description of the Prior Art
A semiconductor switch may be produced by several different junction-forming techniques. Suitable techniques include alloying, diffusion and epitaxial growth methods. Diffusion and alloying have proved to be the most popular methods because they lend themselves to the economical mass production of semiconductor switches. However, both methods result in P-N junctions which are not very sharply defined although they may meet the requirements of a "step" P-N junction. Also, the concentration profile of the P-N junction varies with the process temperature employed. In addition, the diffusion process results in a region having non-uniform impurity concentration throughout the region.
Regions formed by alloying techniques are limited in depth and area covered because the process requires the formation of a molten mixture of the semiconductor material and either a donor or an acceptor material, as an impurity material, and solidification of the same to form the desired region. Also, the molten material must be prevented from flowing outside of a confined area. Otherwise, the device may be inoperable. The material of the region that is formed is recrystallized material. However, the process is limited to that portion of the device immediately adjacent to an electrical contact. Most often, the alloying process is practiced when an electrical contact is affixed to the device.
In either case, the diffusion and alloying techniques are practiced only through a surface area. Because of the elapsed time and high temperature required for the diffusion processes, the resulting regions produced thereby are limited in size. Any additional high temperature processing of the processed body readily affects the P-N junctions which have been previously formed and tends to enlarge the previously formed regions.
Epitaxial growth techniques overcome some of the difficulties encountered by the diffusion and alloying techniques. Regions of fairly uniform resistivity are easily grown. However, the temperatures and the rate of growth of material are factors which limit the thickness that a region can be grown economically. The P-N junction which results from epitaxial growth techniques is more sharply defined than the junctions made by alloying and diffusion.
In all instances, however, theoretical breakdown voltages and operating characteristics are not achieved. High temperatures for extended periods of time adversely affect the crystalline structure of the substrate material. It causes the P-N junction already present in the processed material to move and may also result in the out diffusion of dopant impurities thereby changing the resistivity of the region of semiconductor material involved. Additionally, the processes deposit impurities within or on surfaces of the regions and at the P-N junctions thereby detrimentally affecting the operating characteristics of the devices.
None of the semiconductor switches manufactured to date have several regions made from recrystallized substrate material. In addition, the semiconductor switches have usually been limited to four regions for semiconductor control rectifier applications and to five regions for bidirectional switching applications. Such structural limitations have resulted from extensive process time and high process temperatures encountered in making the devices.
An object of this invention is to provide a new and improved semiconductor switch which overcomes the deficiencies of the prior art devices.
Another object of this invention is to provide a new and improved semiconductor switch having at least one group of four regions of opposite type conductivity, at least three of the regions being of recrystallized semiconductor material having solid solubility of a dopant impurity therein to impart the conductivity type thereto.
Another object of this invention is to provide a new and improved semiconductor switch wherein the P-N junctions formed therein are well defined and have a minimum concentration profile width to provide a step P-N junction.
Other objects of this invention will, in part, appear hereinafter.